Generally, a surveying instrument includes an inclination sensor inside, and is leveled based on detection results of the inclination sensor. As this inclination sensor, a liquid surface reflection type is frequently used.
FIG. 1 is a configuration diagram of an optical system of a liquid surface reflection type inclination sensor. The liquid surface reflection type inclination sensor includes an illumination section 11, a collimating lens 12, a barcode pattern 13, a polarizer 14, a beam splitter 15, a focus lens 16, a λ/4 plate 17, a liquid 18, a container 19, a light receiving element 20, and an arithmetic processing unit 21. In the liquid surface reflection type inclination sensor, the liquid 18 is sealed in the container 19, light is irradiated onto the barcode pattern 13 from the illumination section 11 and reflected by the surface of the liquid 18 (free liquid surface following gravity, hereinafter, referred to as a liquid surface 18′), and this reflected light is received by the light receiving element 20, and a displacement amount of the detection image thereof is detected.
In the liquid surface reflection type inclination sensor, the reflectance of the liquid surface 18′ is low, and most of a light flux incident on the liquid 18 is transmitted through the liquid surface 18′, so that an error caused by an image reflected by the ceiling of the container 19 cannot be ignored in some cases. Therefore, in the inclination sensor described in Patent literature 1, by forming the ceiling of the container 19 into an inclined surface 19a (shown by the dashed line in FIG. 1), the reflected light on the ceiling is designed to deflect and prevent an unnecessary reflected light image from being received by the light receiving element 20.